This application claims the priority of German Patent Application Serial No. 199 52 807.1, filed Nov. 2, 1999, pursuant to 35 U.S.C. 119(a)-(d), the subject matter of which is incorporated herein by reference.
The present invention relates to a method of machining electrically conductive workpieces with a cutting tool. The present invention further relates to the use of a cutting tool for machining electrically conductive workpieces, and to a holding device for a workpiece to be machined.
Machining electrically conductive workpieces is being widely in use. For example, lathes and milling machines are utilized for machining metallic workpieces. Of course plastic workpieces, which can also be electrically conductive, may be machined as well. In conventional machining tools, the workpiece is clamped in a holding device, and subsequently an operator carries out the machining operation. This requires some basic knowledge on the part of the operator about the machining operation, even when processing simple workpieces, and moreover requires a continuous presence of the operator during the machining operation. This type of machining operation is appropriate for single pieces, small-scale production and finishing of workpieces, e.g., turning of already used brake disks and brake drums.
Other approaches involve the use of automated machining devices to execute a previously inputted machining program in the absences of an operator. The use of such machining device is economically sensible only when the machining process involves a large-scale production of identical workpieces or complicated special parts.
In particular, when finishing operations of workpieces are involved in order to smoothen the surface, an operator is typically required to provide an optical check of the finished surface so as to ascertain whether the finishing process is completed or needs to be continued. Hereby, it becomes oftentimes necessary to remove the workpiece from the holding device. This is time-consuming and not satisfactory because it frequently happens that existing surface flaws cannot be detected as a consequence of bad light conditions.
German Pat. No. 10 85 009 C describes a device for extending the service life of hard metal tools used in machine tools, whereby thermoelectric currents are compensated at the point of machining between the workpiece and the tool.
U.S. Pat. No. 5,802,937 describes a method for machining surfaces in the nanometer range by applying a closed electric circuit between workpiece and cutting tool and controlling the depth of cut of the cutting tool by compensating a difference between actual value and desired value in the electric circuit.
German Pat. No. 196 33 942 A1 describes a holding device, in particular for a cutter blade, for use in finishing machines and/or machine tools. The holding device is made of electrically conductive material, with at least part of its surface provided with an insulating coating so that an electrical contact is provided between the cutter blade and the holding device to thereby ascertain an engagement of the cutter blade in a workpiece.
It would be desirable and advantageous to provide an improved method to terminate a machining operation of a workpiece surface as soon as a desired surface quality has been realized.
According to one aspect of the present invention, a method for machining an electrically conductive workpiece with a cutting tool, includes the steps of closing an electric circuit between the tool and the workpiece which is positioned electrically insulated from the tool, when the workpiece is subjected to a machining operation, determining and evaluating at least one electric variable in the electric circuit during the machining operation, advancing the workpiece after each machining step of an entire surface of the workpiece, and terminating the machining operation, when the at least one variable remains substantially constant during a machining step.
The present invention resolves prior art problems by applying an electric circuit between a cutting tool, e.g. a turning tool, and a workpiece to be machined and recognizing that a jump in resistance occurs in response to an area of irregularities in the workpiece surface being machined so that signals fed back during a machining step across the entire surface provide a reflection of the surface quality at that time. The degree of the surface quality can be calibrated beforehand and is commensurate with a degree of constancy of the feedback signal. As soon as the desired surface quality has been reached, the machining operation is terminated.
According to another feature of the present invention, the feedback signal may be a voltage signal, current strength signal or a resistance signal and is determined in the closed electric circuit in response to an electric input voltage or input current strength. Suitably, the input current strength is up to 50 mA. The input voltage may be up to 24 V. The electric current may be applied in an area of the cutting tool or in an area of a holding device of the workpiece. Likewise, the determination of the feedback signal may be realized in an area of the cutting tool or in an area of a holding device of the workpiece.
The method according to the present invention is applicable for machining a cylindrical workpiece, especially brake disks and brake drums.
A cutting tool according to the present invention for machining an electrically conductive workpiece, may be operatively connected in an area of the cutting tool to a current-application device for applying an electric current and/or a device for determining and evaluating at least one electric variable.
According to another aspect of the present invention, a holding device may be used for a workpiece to be machined, whereby a current-application device for applying an electric current and/or a device for determining and evaluating at least one electric variable may be provided in the area of the holding device.
With a method according to the present invention, it is now possible to ascertain with high precision any presence of surface irregularities, such as flutes, of the workpiece during the machining operation and to directly control the machining process in response to the actual condition of the workpiece surface. In particular, it is now possible to automatically terminate the machining process, in the absence of an operator, as soon as the desired surface quality has been accomplished.
Moreover, the method according to the present invention affords the possibility to protect the machine tool through determination of an electric variable, which responds to an electrically conductive body, so that the machine tool can be automatically shutdown or an inquiry can be triggered in the presence of such an electrically conductive body in an unexpected area, e.g. in an area in which the cutting tool is intended to travel.